1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof.
In this specification, a semiconductor device means all types of devices that can function by utilizing semiconductor characteristics, and a transistor, a semiconductor circuit, a memory device, an imaging device, a display device, an electro-optical device, an electronic device, and the like are all semiconductor devices.
2. Description of the Related Art
In recent years, transistors that are formed using a semiconductor thin film having a thickness of several nanometers to several hundreds of nanometers over a substrate having an insulating surface such as a glass substrate have been attracting attentions. Transistors are widely used for electronic devices such as ICs (integrated circuits) and electro-optical devices. In particular, transistors are urgently developed as switching elements of image display devices typified by active matrix liquid crystal display devices, active matrix EL (electroluminescent) display devices, and the like. In an active matrix liquid crystal display device, a voltage is applied between a pixel electrode connected to a selected switching element and a counter electrode corresponding to the pixel electrode, and thus, a liquid crystal layer disposed between the pixel electrode and the counter electrode is modulated optically. The optical modulation is recognized as a display pattern by an observer. An active matrix display device here means a display device which employs a method in which a display pattern is formed on a screen by driving pixel electrodes arranged in matrix using switching elements.
The range of uses of such an active matrix display device is expanding, and demands for larger screen size, higher definition, and higher aperture ratio are increasing. In addition, it is demanded that the active matrix display device has high reliability and that a production method of the active matrix display device offers high productivity and reduces production cost. Simplification of a process is one way for increasing productivity and reducing production cost.
In active matrix display devices, transistors are mainly used as switching elements. In manufacturing transistors, reduction in the number of photolithography processes or simplification of the photolithography process is important for simplification of the whole process. For example, when one mask for photolithography process is added, the following steps are further needed: resist application, prebaking, light exposure, development, postbaking, and the like and, moreover, steps before and after the aforementioned steps, such as film formation, etching, resist removal, cleaning, drying, and the like. The number of steps is significantly increased only by adding one mask for photolithography process in the manufacturing process. Therefore, many techniques for reducing the number of photolithography processes or simplifying the photolithography process in a manufacturing process have been developed.
Transistors are broadly classified into top-gate transistors, in which a channel formation region is provided below a gate electrode, and bottom-gate transistors, in which a channel formation region is provided above a gate electrode. These transistors are generally manufactured through at least 5 photolithography processes using at least 5 photomasks.
In an active matrix EL display device, a partition layer for separating EL layers for respective pixels is necessary. Therefore, it is general to manufacture an active matrix EL display device using one more photomask, that is, through at least 6 photolithography processes.
Many conventional techniques for simplifying the photolithography process use a complicated technique such as backside light exposure, resist reflow, or a lift-off method, which requires a special apparatus in many cases. Using such complicated techniques may cause various problems, thereby leading to a reduction in yield. Moreover, the usage of such complicated techniques often cause deterioration of electrical characteristics of transistors.
As typical means for simplifying the photolithography process in a manufacturing process of a transistor, a technique using a multi-tone mask (called a half-tone mask or a gray-tone mask) is widely known. As a technique for reducing the number of manufacturing steps by using a multi-tone mask, Patent Document 1 can be, for example, given.